1. Field of the Invention
The present invention relates generally to power tools, and more particularly, to a quick-release depth adjustment device for a power tool.
2. Description of the Related Art
A general power tool is powered by an electric motor or pneumatic turbo for rotary output to drive rotation of a drive bit or drill bit. As far as the drive bit is concerned, the drive bit is usually connected with a power source by a clutch in such a way that the user must hold the power tool and apply a positive force to the drive bit to enable the drive bit to be coupled with the power source and then to be driven by the power source for rotation.
When the drive bit screws a screw onto a workpiece, the screw must be stopped from rotation driven by the drive bit after contact with the surface of the workpiece or while the screw is screwed tight to enable allowable recession formed on the surface of the workpiece. Otherwise, if the drive bit keeps rotation, the screw head may be damaged or the screw may damage the workpiece, stripping the screw. However, the user operating the power tool having incessant power output often fails to stop operation of the power tool immediately at the right time and the maximum torsion of the clutch is often great enough to damage the screw or the workpiece.
To solve the aforementioned problem, a variety of conventional depth adjustment devices for power tools were proposed to stop the drive bit from moving further after the screw is screwed to a predetermined depth of the workpiece and to interrupt the transmission coupling between the drive bit and the power tool to prevent the drive bit from tightening the screw. For example, each of U.S. Pat. Nos. 4,030,383, 4,647,260, and 5,341,704 similarly discloses that a length-adjustable sleeve is fixed to a front end of the power tool having a clutch for the drive bit to run through and the drive bit is exposed outside for a predetermined length mating the depth where the screw is screwed into the workpiece. When the screw is screwed into the workpiece for a predetermined depth, a front end of the sleeve contacts the surface of the workpiece to disable the user from keeping applying the positive force to the drive bit via the power tool to interrupt the power coupling of the clutch. In this way, the power tool stops driving rotation of the drive bit to prevent the screw from threadably moving further while the screw is screwed to the predetermined depth, thus preventing the screw from being damaged by the drive bit or from damaging the workpiece.
Although the aforesaid conventional depth adjustment devices provide the function of depth adjustment, the way that each of those devices is connected with the power tool is disadvantageous to assembly and disassembly of the depth adjustment device. For example, each of the depth adjustment devices of the U.S. Pat. Nos. 4,030,383 and 4,647,260 is mounted to the front end of the power tool by screw thread fit, such that the assembly and disassembly of the depth adjustment device are not convenient. Particularly, the power tool itself often needs to mate with different apparatuses for operation, such that the aforesaid device mounted to the front end of the power tool must be replaced frequently. For example, an automatic screw feeding device instead of the depth adjustment device is provided for supplying screws quickly and uninterruptedly. However, in the automatic screw feeding device, the screw feeding track must be fixed to a specific location, such that the screw thread fit is not applicable to the connection between the automatic screw feeding device and the power tool. In this way, the power tool can only be installed with one of the devices. The depth adjustment device of the U.S. Pat. No. 5,341,704 is mounted to the power tool by that its annular rib formed inside the sleeve is engaged with the annular grove formed at the front end of the power tool. Such design is subject to wear and tear of the annular rib/groove during the assembly and the disassembly and thus disadvantageous to repeated assembly and disassembly.